1. Field of the Invention
The present invention relates to a communication system for an array of direct access storage devices or DASD; and more particularly, a communication system which provides for bypassing one and/or multiple DASD.
2. Description of Related Art
SCSI buses such as in a serial storage architecture SSA are normally wired in a loop configuration. Specifically, the four wire, two differential pairs, full duplex cable is wired into a DASD port 1, through the DASD, out the DASD port 2, and on to the next DASD. The originating end of the cable comes from an SSA DASD controller card or initiator, and returns via the last DASD in the loop or communication path to the initiator.
FIG. 1 illustrates such a prior art communication system for an array of DASD. As shown in FIG. 1, a computer 81 includes an operating system 82 in communication with an initiator 80. The communication system also includes a DASD array 86 of DASD slots 88. The DASD slots 88 are daisy-chained with the first DASD slot 88 and the last DASD slot 88 connected to two different ports of the initiator 80 to form a communication path. The operating system 82 also communicates with each DASD slot 88 via a network 84.
Each DASD slot 88 includes a DASD tray 24. The DASD tray 24 includes a DASD 18 and a regulator 16. As shown, the regulator 16 supplies power to the DASD 18. It is with the regulator 16 that the operating system 82 communicates via the bus 84.
During operation, the operating system instructs each regulator 16 as to whether the regulator 16 should supply power to an associated DASD 18. Also during operation, the initiator 80 implements a communication protocol by which data is transferred to and from the DASD array 86.
Because of the serial arrangement of the DASD array 86, if a DASD tray 24 is not present in a DASD slot 88, the communication path is broken. In order to maintain the communication path when a DASD tray 24 is not present, an operator must insert a dummy DASD tray in the empty DASD slot 88. The dummy DASD tray is merely a connector or jumper which serves to connect the previous DASD slot 88 with the succeeding DASD slot 88. Additionally, if a DASD 18 is to be added to the DASD array 86, user intervention is required to maintain a properly connected loop or communication path.
If a DASD 18 should malfunction, then the communication path or loop is broken. Using the communication protocol, the initiator 80 establishes which of the DASD slots 88 contains the malfunctioning DASD 18. The operating system 82 then causes the regulator 16 associated with the malfunctioning DASD 18 to power down the malfunctioning DASD 18, and the operating system 82 communicates this information to a user. A user must then remove the DASD tray 24 containing the malfunctioning DASD 18, and replace the DASD tray 24 with a dummy DASD tray or a new DASD tray 24. Only then is the communication path reconnected and normal un-degraded SCSI operation resumed.